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Valid Saturday August 26, 2017 to Wednesday September 06, 2017
US Hazards Outlook
NWS Climate Prediction Center College Park MD
EDT August 23 2017Synopsis
: Tropical Depression Harvey is
forecast to intensify and then move inland into southeast Texas on August 26.
Its remnant low is expected to remain across the western Gulf Coast region
through early next week. A tropical cyclone may develop near the Southeast
coastline early next week with a subsequent track to the northeast. Upper-level
high pressure is forecast to build across the western U.S. during the final
week of August. Upper-level low pressure is likely to remain centered over
western Alaska and the Aleutians through Week-2. Hazards
Detailed Summary For Saturday August 26 - Wednesday August 30:
- Heavy rain and flooding for the Texas Gulf Coast, inland areas of
southeast Texas, and the lower Mississippi Valley, Sat-Wed, Aug 26-30.
- Significant waves and storm surge along the Texas Gulf Coast, Sat, Aug 26.
- High winds for the Texas Gulf Coast, Sat, Aug 26.
- Heavy rain for parts of central and southern Florida, Sat, Aug 26.
- High winds for the Outer Banks of North Carolina, Mon-Tue, Aug 28-29.
- Much above-normal temperatures for parts of the Pacific Northwest, northern
Great Basin, and northern Rockies, Sun-Wed, Aug 27-30.
- Heavy rain for the Alaska Peninsula, Kenai Peninsula, and Prince William
Sound, Mon-Wed, Aug 28-30.
- Moderate risk of much above-normal temperatures for parts of Idaho, Nevada,
Oregon, and Washington, Thu, Aug 31.
- Slight risk of much above-normal temperatures for much of the western U.S.,
Thu-Wed, Aug 31-Sep 6.
- Flash flooding possible for the desert Southwest, Thu-Wed, Aug 31-Sep 6.
- Flooding likely for parts of northwest New Mexico.
- Flooding occurring, imminent, likely, or possible for parts of Kansas and
- Severe Drought across parts of the Great Plains, western Corn Belt,
northern Rockies, and Hawaii.
As of 2pm
EDT on Aug 23, Tropical Depression Harvey is located over the Bay of Campeche.
Harvey is likely to intensify prior to this period and could become a hurricane
when it makes landfall along the Texas Gulf Coast later this week. Based on the
latest model solutions, high winds (potentially hurricane force) and storm
surge are forecast along the Texas Gulf Coast through Aug 26. After making
landfall, the remnant low is expected to move erratically across the western
Gulf region. The 6Z GFS and 0Z ECMWF ensemble means are in better agreement
today, maintaining a remnant low near the western Gulf Coast through early next
week. Therefore, a prolonged period of heavy rainfall and flooding is likely
for the Texas Gulf Coast and lower Mississippi Valley through August 30. The
deterministic 0Z ECMWF model indicates 7 to 10 inches, locally more, of
rainfall from Aug 26 to 30 across the outlined heavy rain area. Maximum
rainfall amounts may exceed 15 inches across southeast Texas and southwest
Louisiana with an increasing risk of major flooding. Please refer to the
National Hurricane Center and local NWS offices for the latest updates and
A trough of low pressure and enhanced moisture are expected to bring heavy
rain (locally more than 2 inches) to parts of central and south Florida on Aug
26. Deterministic model solutions continue to indicate slow tropical or
subtropical cyclone development offshore of the Southeast coastline this
weekend. An increasing pressure gradient between this area of low pressure and
a 1028-hpa surface high over New England is expected to result in high winds
(gusts above 35 knots) across the Outer Banks on Aug 28 and 29.
An upper-level ridge is forecast to strengthen across the western U.S. with
500-hpa heights increasing to more than 588dm early in this period. Much
above-normal temperatures are posted for areas of the Pacific Northwest,
northern Great Basin, and northern Rockies where maximum temperatures are
forecast to average around 12 degrees F above normal and exceed 90 degrees F.
Also, numerous wildfires continue to burn in these same areas with the largest
wildfire (more than 68,000 acres burned) at Nena Springs in northern Oregon.
Anomalous southerly flow with a tropical connection is expected to affect
southern mainland Alaska next week as a highly amplified trough (ridge)
develops over the Aleutians (eastern Alaska). The anomalous southerly, onshore
flow is likely to bring heavy rain (more than 1.5 inches per 24 hours) from the
Alaska Peninsula east to the Prince William Sound. The 0Z deterministic ECMWF
model indicates a 72-hour maximum of more than 10 inches along the southern
Kenai Peninsula from Aug 28 to 30. For Thursday August 31 -
Wednesday September 06:
The remnant low from Tropical Depression Harvey is
expected to lift away from the Gulf Coast at the beginning of Week-2. Although
there is support for an area of heavy rain somewhere across the eastern U.S. on
day 8, the exact location varies largely among model guidance. The 0Z ECMWF
ensemble mean indicates generally less than 0.25 inch across much of the
eastern U.S. on Aug 31, representative of the large ensemble spread. Also, the
area for heaviest rain varies among recent GFS ensemble mean solutions.
Therefore, a categorical heavy rain hazard is not posted beyond day 7.
The GFS and ECMWF ensemble means maintain a stable longwave pattern through
the beginning of September with an upper-level ridge (trough) over western
(eastern North America). The most anomalous longwave trough across the Northern
Hemisphere remains the upper-level trough extending from the Bering Sea to
south of the Aleutians. The amplified upper-level ridge supports a slight (Aug
31-Sep 6) to moderate (Aug 31) risk of much above-normal temperatures across
the western U.S.
Tropical cyclone (TC) development is favored across the east Pacific by the
end of August. A northwest track of a TC close to the Baja peninsula is
expected to bring a surge of moisture north from the Gulf of California into
the desert Southwest during early September. Therefore, a flash flooding
possible hazard is posted for the desert Southwest.
The U.S. Drought Monitor valid on Aug 15 indicates that severe to
exceptional (D2-D4) drought coverage decreased slightly from 5.38 percent to
5.33 percent across the continental U.S.
Forecaster: Brad Pugh
Click here to see a display of the GFS Ensemble Forecasts
Please consult local NWS Forecast Offices for short range forecasts and region-specific information.